diff -r 56afb5950289 -r e5afb97b54f6 Fahad/Eclipse/ScalaProjects/bin/POSIX/PosixAlgo.sc
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Fahad/Eclipse/ScalaProjects/bin/POSIX/PosixAlgo.sc	Tue Nov 04 19:51:39 2014 +0000
@@ -0,0 +1,44 @@
+package POSIX
+
+object PosixAlgo {
+
+  abstract class Rexp
+  case object NULL extends Rexp
+  case object EMPTY extends Rexp
+  case class CHAR(c: Char) extends Rexp
+  case class ALT(r1: Rexp, r2: Rexp) extends Rexp
+  case class SEQ(r1: Rexp, r2: Rexp) extends Rexp
+  case class STAR(r: Rexp) extends Rexp
+  case class RECD(x: String, r: Rexp) extends Rexp
+
+	abstract class Val
+  case object Void extends Val
+  case class Chr(c: Char) extends Val
+  case class Sequ(v1: Val, v2: Val) extends Val
+  case class Left(v: Val) extends Val
+  case class Right(v: Val) extends Val
+  case class Stars(vs: List[Val]) extends Val
+  case class Rec(x: String, v: Val) extends Val
+
+	def nullable(r: Rexp): Boolean = r match {
+    case NULL => false
+    case EMPTY => true
+    case CHAR(_) => false
+    case ALT(r1, r2) => nullable(r1) || nullable(r2)
+    case SEQ(r1, r2) => nullable(r1) && nullable(r2)
+    case STAR(_) => true
+    case RECD(_, r1) => nullable(r1)
+  }                                               //> nullable: (r: POSIX.PosixAlgo.Rexp)Boolean
+  
+  def der(c: Char, r: Rexp): Rexp = r match {
+    case NULL => NULL
+    case EMPTY => NULL
+    case CHAR(d) => if (c == d) EMPTY else NULL
+    case ALT(r1, r2) => ALT(der(c, r1), der(c, r2))
+    case SEQ(r1, r2) =>
+      if (nullable(r1)) ALT(SEQ(der(c, r1), r2), der(c, r2))
+      else SEQ(der(c, r1), r2)
+    case STAR(r) => SEQ(der(c, r), STAR(r))
+    case RECD(_, r1) => der(c, r1)
+  }                                               //> der: (c: Char, r: POSIX.PosixAlgo.Rexp)POSIX.PosixAlgo.Rexp
+}
\ No newline at end of file